Compander systems comprise basically a compression circuit for the dynamic narrowing of a useful signal such e.g. as a LF voltage prior to transmission or storage and and expansion circuit for dynamic widening after the transmission or prior to the reproduction of a previously stored signal. The adjusting members frequently provided for such compander systems of the above-mentioned type serve to reduce or increase the signal amplitude and/or to influence the frequency of the signal to be dealt with, the control signal necessary for controlling the adjusting member being derived from the input or output signal of the compander circuit.
A corresponding system is known from U.S. Pat. No. 3,969,680. In such a system there is provided in a useful signal path a negatively back-coupled amplifier for the useful signal the inverting input of which is connected via a negative feedback path including an electronically controllable impedance to the output of the amplifier. From the output or input of the useful signal path there is branched off a path in which there is derived from the useful signal a control voltage which is applied to the electronically controllable impedance.
Moreover from the German Offenlegungsschrift 22 18 823 and U.S. Pat. No. 3,815,039 it is known, in an amplifier which can be changed over between two operating conditions, to effect the change of amplification by means of a controllable impedance which in the one operating condition of the amplifier is connected to a negative feedback path of the amplifier and in the other operating condition is connected to the input of the amplifier.
The circuits provided for the generation of the control voltage for the controllable negative feedback path of a voltage divider have as a rule unavoidable time constants, hence cause delays of the control voltage. There is the possibility that upon a rapid rise of the useful input signal the change of amplification of the said amplifier that in itself is necessary cannot follow this rise and therefore a so-called overswing occurs. In this manner there occurs at the output of the compander system, during compression, a momentarily excessively high output signal. A corresponding likewise undesired effect occurs during expansion when the amplitude of the useful input signal changes very rapidly.
For the avoidance of the disadvantage described one might think of limiting that voltage which in the case of compression exists over the negative feedback path and in the case of expansion exists over the corresponding partial impedance of the said voltage divider. This proposal is the subject of German Offenlegungsschrift 26 19 192 and U.S. Pat. No. 4,119,922.
It has been found that difficulties arise if for the limiting switch means there are employed components or circuit arrangements of which the threshold value at which the limitation begins is equal to or higher than the maximum permissible voltage on those impedances the voltage drop of which is to be limited.